Home cells, of which femto cells and Wi-Fi access points are examples, sometimes known as an access point base station are typically designed for use in residential or small business environments.
A femtocell is a low-power wireless access point that operates in licensed spectrum and connects mobile devices to a mobile network using a residential broadband Internet connection. Depending on the network operator and mobile device capability, a mobile device may use a variety of different radio access technologies (RAT)—for example one or more 3GPP2-defined cdma2000 1x/1xEV-DO and/or one or more of 3GPP-defined air interfaces such as GSM/GPRS/EDGE, WCDMA, TD-SCDMA, LTE etc. In 3GPP terminology, a home cell is referred to as Home Node B or enhanced Home Node B. A femtocell incorporates functionality of a typical base station—for example a UMTS femtocell may comprise a Node B, a Radio Network Controller (RNC), a Serving GPRS Support Node (SGSN) that uses subscriber's cable or DSL connection for backhaul connectivity. It allows service providers to extend or provide better network coverage indoors, especially where network coverage would otherwise be limited or unavailable to mobile devices. Femtocells are typically overlaid by one or more macro cells and serve a significantly smaller geographic area or subscriber constituency than a macro cell.
A Wi-Fi access point is another low-power wireless access technology that operates on un-licensed bands which is defined by IEEE 802.11 standards. In IEEE 802.11 standards, a home cell is referred as an access point. A mobile device that can operate in air interfaces defined by multiple standard bodies is typically called a multi-mode device—for example, a multi-mode device may be capable of supporting cdma2000 1x/1xEV-DO, EDGE/HSPA/LTE in addition to WiFi.
Typically home cells do not broadcast any indication identifying it as a home cell. For example, a UMTS femtocell that is not compliant with 3GPP release 8 or above will not have such indication. Even if a femtocell (such as a 3GPP release 8 compliant cell) that may provide such indication over a broadcast channel, a UMTS mobile device not compliant with release 8 will not be able to decode such indication. In other words, there is a variety of situations when a mobile device may not be able to distinguish between a typical macro cell and a femtocell including his/her own. Even if a femtocell explicitly identifies itself as a femto cell, the presence or proximity of this femto cell may not be broadcast in the macro cells due to limits on neighbour cell information that the macro cell can broadcast. Currently, an UMTS cell can have at most thirty two neighbour cells information on a given frequency, whereas a macro cell can have tens if not hundreds of femto cells overlapped in a coverage area. Considering this limitation, the mobile device would not have information regarding a target femto cell presence and therefore would not know when or where to initiate a scan for the targeted femtocell in an intelligent manner.
On the other hand, the preferred network/cell for a femtocell owner/subscriber is their own or subscribed femtocell(s) for a variety of reasons such as lower or better tariff on usage. Therefore a typical implementation of a mobile device that operates both in an operator's macro network and subscribed femtocells involves frequent searching for more preferred femtocell which in turn may lead to increased consumption of battery power and reduction in battery life. A similar scenario may exist if the home cell belongs to another RAT (radio access technology) that does not belong to the same core network, as for example where the cellular network that the mobile device is currently on is part of a UMTS network and the home cell is a WiFi network. There may not be coordination between multiple air-interfaces and underlying networks in terms of broadcasting presence of one another.